The present invention relates to a scanner that illuminates light onto a document and reads in the reflected light. More particularly, the present invention relates to a scanner that has a cold-cathode-tube light source used as a reading light source and a temperature control circuit for the light source.
Conventionally, some scanners of the type, each which illuminates a document and reads the reflected light, are utilized as scanners for facsimile machines or singly as handy type scanners. Light-emitting diode (LED) light sources or cold-cathode-tube light sources are used as the light source for the scanner. The cold-cathode-tube light source can provide a high luminance (brightness) with low power consumption. This light source is effective for high-speed color scanners.
On the other hand, JP-A No. 67485/1999 discloses a color liquid crystal display as a device including a cold-cathode-tube light source. The cold-cathode-tube light source is used as the back light source for color liquid crystal devices.
Generally, there is the problem in that the cold-cathode-tube light source has the disadvantage in that a change in temperature of the saturated vapor pressure of mercury (Hg) within the cold cathode tube causes a change of luminance. The long use of the liquid crystal display, that is, the long lighting time of the cold cathode tube light source strengthens an influence of a temperature rise due to the self heat dissipation from the cold-cathode-tube light source. For that reason, it is necessary to perform the temperature control by detecting the temperature of the cold-cathode-tube light source as nearly as possible to the cold cathode tube.
In other words, for the cold-cathode-tube light source used as the back light for a liquid crystal display, the scheme of controlling the luminance of the back light according to the temperature of the cold-cathode-tube light source is generally used. In this case, the temperature sensor is disposed near the cold-cathode-tube light source.
However, in the case of a scanner, particularly a small handy-type scanner, it is difficult to dispose the temperature sensor near the cold-cathode-tube light source because of the limited assembly space for the control circuit.
Moreover, in the case of the system of lighting the light source only when the scanner reads a document, the lighting is for a short time of several ten seconds so that the temperature change due to the heat generation of the cold cathode tube itself is small. Hence, in conventional scanners, the temperature rise of the cold-cathode-tube light source is negligible and the temperature control is not performed to the cold-cathode-tube light source.
However, in the case of scanners, when the ambient temperature changes at the lighting start time of the cold-cathode-tube light source (that is, every time of starting a reading operation), the luminance of the cold-cathode-tube light source changes. As a result, the ambient temperature changes the magnitude of an image output signal read out. Particularly, there is the disadvantage in that since a low ambient temperature causes a small magnitude of the image output signal, the S/N ratio of the image output signal is deteriorated, thus degrading the image quality.
In the conventional scanner, a small current is supplied to the cold-cathode-tube light source during non-lighting period to prevent the temperature inside the cold cathode tube from being decreased. However, since the current is continuously flown during non-lighting period, that is, while the document is not being read, the power consumption increases.
The objective of the present invention is to solve the above-described tasks.
Also, the objective of the present invention is to provide a scanner that can maintain the luminance of the cold cathode tube to a constant level. In order to realize good efficiency in a simplified structure, the scanner controls the tube current by detecting only the ambient temperature under actual scanner use conditions.
According to the present invention, a scanner comprises a cold-cathode-tube light source for illuminating a surface of a document; a photoelectric conversion element for receiving light reflected from the surface of the document and producing an image signal; a temperature detection circuit for detecting an ambient temperature; and a control circuit for controlling a drive signal according to detected temperature information, the drive signal illuminating the cold-cathode-tube light source when the document is read.
According to the present invention, a method of controlling a drive signal for illuminating a cold-cathode-tube light source comprising the steps of: detecting an ambient temperature and controlling a drive signal based on said detected temperature information, said drive signal illuminating said cold-cathode-tube light source when said document is read.
In the drive signal control, the current (tube current), voltage, or frequency of the drive signal is controlled. Moreover, the temperature detection circuit and the control circuit are mounted on the circuit board for the existing document reader. This allows temperature control to be realized by adding a minimum number of components, without adding a complicated control circuit.
According to another aspect of the present invention, a scanner comprises a cold-cathode-tube light source for illuminating a surface of a document; a photoelectric conversion element for receiving light reflected from the surface of the document and producing an image signal; an impedance detection circuit for detecting an impedance between electrodes of the cold-cathode-tube light source; and a control circuit for controlling a drive signal according to detected impedance information, the drive signal illuminating the cold-cathode-tube light source when the document is read.
The impedance between electrodes of the cold-cathode-tube light source changes with ambient temperatures. The luminance can be controlled constant by controlling the drive signal of the cold-cathode-tube light source with the detected impedance.